In the field of automobile manufacture, hemming is the process of folding over the edge of a body panel to form a finished edge. Hemming is also used to join two body panels together, such as the inner and outer panels of a door. The hem provides a secure mechanical joint, and the resulting edge of the hemmed joint is neat and finished.
The lower edge of a window opening in a vehicle door panel is called the beltline. A reinforcing plate, normally called a beltline reinforcement, is usually attached to the inside of the outer panel along the beltline. The beltline reinforcement provides added strength to the door in the event of a side impact. Until the present time, the beltline reinforcement was attached to the outer panel by a series of spot welds. Although spot welds provide a secure mechanical connection between the beltline reinforcement and the outer panel, the welds show through on the Class A surface of the outer panel. As automobile fit and finish has become more refined, it has become increasingly important to mask over or otherwise obliterate any marks or indentations caused by the spot welding process so that they will not be visible on the outside panel surface. The normal solution is to cover welds of the beltline joint with a piece of trim material that is usually in the form of a chrome or rubber strip.
Hemming would be a desirable way to join the beltline reinforcement to the outer panel, since the hemming process inherently leaves the outer panel without impressions or marks that need to be masked over with trim material. A beltline hemming mechanism would be less expensive than weld guns, and if the beltline hemming could be performed at the same time as other hemming operations were being performed on the same panel, the overall processing time for the panel could be reduced. Until recently, hemming has not been used to join the beltline reinforcement to the outer panel, and there are various reasons for this. If hemming is being used to hem the inside perimeter of the window opening, the space in the window opening is already occupied by that hemming mechanism, and there is not enough room for a separate beltline hemming machine. If the beltline hemming mechanism is combined with the mechanism for hemming some other portion of the window opening or the door panel, the timing and synchronization of the motion of all of the hemming steels, if they are driven by a single actuator, becomes an almost impossible task, and it is very difficult to set-up the hemming steels so that they all reach a Final Hem Complete position at the same time. Even if the steels are perfectly timed relative to one another, any variation in the thickness of the workpiece in a production run, or in the stroke or timing of the steels, results in a final hem that is not completely closed, or a hemming mechanism that becomes overly stressed because it is being driven past the Final Hem Complete position. One solution is to provide a second hemming machine at its own station in order to do the beltline hemming. This eliminates the timing and synchronization issues between the beltline hemming steels and the other hemming steels and allows the beltline hemming machine to be set up and tuned independently of the other hemming machines. The second hemming machine requires additional floor space however, and, because a second dedicated drive is required, the second hemming machine represents a substantial additional cost.
It would accordingly be desirable to provide a hemming machine that would hem the beltline of a vehicle door panel in the same station that is performing other hemming on the panel, using a single machine and a single drive mechanism, but would allow the hemming tools to finish their respective hemming strokes at different times.